The present invention refers to a piston ring for an internal combustion engine, more particularly, a piston ring having coatings of Chromium Nitride and Niobium Nitride, to confer greater resistance to wear of the ring during the useful life of the engine.
As a rule, the wear process that a material may sustain or cause, in contact with another, is a rather complex phenomenon. The tribological properties of the material are one of the main characteristics involved in this process.
The tribological properties are related to wear and tear and to the resistance of a material, when it is subject to frictional forces.
There is currently a growing demand for materials and/or components that offer greater resistance to wear and to corrosion, which has fostered interest for the area of surface engineering.
As far as the automobile sector is concerned, and more specifically, to the coating of piston rings of combustion engines, recent studies and research are being developed with a view to achieving materials that offer greater resistance to wear and tear, thus avoiding the rupture/breakage of the coating material, which would severely compromise its functional requirement.
Combustion engine piston compression rings are used to offset the inevitable gap existing between the piston and the cylinder wall, providing an efficient sealing for the combination when the engine is operating (chiefly when it compresses the air or air/fuel mix), in addition to enhanced performance. In this case, the ring avoids combustion gases from passing from the cylinder to the sump, besides preventing the lubricant from accessing the combustion chamber.
There are also oil-scraping rings having various constitutions and functions, which will not be detailed in this specification.
Due to the location of the compression rings in the piston and to the type of force to which they are submitted, among other factors, the rings undergo severe wear and tear processes, and chemical attacks from sub-products from the burning of the air-fuel mixture.
Today, a surprising enhancement in the efficiency of internal combustion engines is occurring, which generate ever greater potency with lesser displacements. Said increase in efficiency is particularly the result of the ever more severe laws limiting the emission of pollutants and the need to reduce the specific consumption of engines, and it is more fashionable than ever due to the increasingly talked of subject of global warning coming from the emission of carbon due to fossil fuel burning.
Some recent works and research have been designed to developed products that find a greater resistance to wear and tear, normally applying a coating and/or a treatment on the surface of the piston rings.
U.S. Pat. No. 5,743,536 describes a piston ring for an internal combustion engine, whose surface is coated by a friction material mainly comprised of Chromium Nitride. The coating is formed by the deposit of chromium nitride on the surface of the part desirable to apply using a process called PVD or physical vapor deposition.
The invention described in this document suggests the use of Chromium Nitride in the form CrN or Cr2N. The proposed coating can also comprise a mixture of CrN and Cr2N.
Further according to the U.S. Pat. No. 5,743,536, the coating is more resistant to the delamination of the surface of the ring, and to abrasion. The coating has a columnar crystalline structure thickness of from 1 to 80 μm, and porosity in the range of 1.5% to 20%.
U.S. Pat. No. 5,316,321 describes a piston ring for internal combustion engine, made from a titanium alloy, whose outer friction surface is coated by a hard film, using the process of physical vapor deposition for this deposition.
U.S. Pat. No. 5,316,321 also describes the use of one or more types of films for the coating of the piston ring, especially TiN (Titanium Nitride) and CrN (Chromium Nitride).
U.S. Pat. No. 5,154,433 describes a piston ring for internal combustion engine, and more particularly, a piston ring made of martensitic stainless steel coated with a Titanium Nitride film. According to one of the embodiments of the invention described in document U.S. Pat. No. 5,154,433, the coating of Titanium Nitride has a concentration of Nitrogen gradually decreasing on its surface, which confers the film a Vickers hardness of around 1800 HV or less.
US 2005/0218603 describes a piston ring for internal combustion engine made of steel, whose contact surface has a coating of aluminum nitride and/or silicone and/or zircon. The proposed coating presents a thickness less or equal to 70 μm, and is applied by physical vapor deposition.
It is noted, from the state of the art documents, that the Chromium Nitride film is widely used in the composition of an outer coating for the combustion engine piston ring, in order to confer greater resistance to wear and tear.
Recently, however, it was noted that in engines working in a high load regime, or engines working under high combustion pressure levels, degradation of the Chromium Nitride layer occurs, with the consequent formation of micro-cracks on their surface. The microcracks propagate until the covering starts to peel (a phenomenon known as delamination). The hard particles, resulting from delamination, loose in the system, may lead to severe wear of the piston ring and to a catastrophic failure of the engine.
Thus, the present invention offers a solution for the problem of delamination of the covering and greater resistance to wear and tear, proposing a piston ring comprised by an outer metallic coating, including Chromium Nitride, in the forms CrN or Cr2N, and Niobium Nitride, in the forms NbN or Nb4N3.